Transcript
STRINGS AND PATTERN MATCHING • Brute Force, Rabin-Karp, Knuth-Morris-Pratt
What’s up? I’m looking for some string. That’s quite a trick considering that you have no eyes. Oh yeah? Have you seen your writing? It looks like an EKG!
Strings and Pattern Matching
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String Searching • The previous slide is not a great example of what is meant by “String Searching.” Nor is it meant to ridicule people without eyes.... • The object of string searching is to find the location of a specific text pattern within a larger body of text (e.g., a sentence, a paragraph, a book, etc.). • As with most algorithms, the main considerations for string searching are speed and efficiency. • There are a number of string searching algorithms in existence today, but the two we shall review are Brute Force and Rabin-Karp.
Strings and Pattern Matching
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Brute Force • The Brute Force algorithm compares the pattern to the text, one character at a time, until unmatching characters are found: TWO ROADS ROADS TWO ROADS ROADS TWO ROADS ROADS TWO ROADS ROADS TWO ROADS ROADS
DIVERGED IN A YELLOW WOOD DIVERGED IN A YELLOW WOOD DIVERGED IN A YELLOW WOOD DIVERGED IN A YELLOW WOOD DIVERGED IN A YELLOW WOOD
- Compared characters are italicized. - Correct matches are in boldface type. • The algorithm can be designed to stop on either the first occurrence of the pattern, or upon reaching the end of the text.
Strings and Pattern Matching
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Brute Force Pseudo-Code • Here’s the pseudo-code do if (text letter == pattern letter) compare next letter of pattern to next letter of text else move pattern down text by one letter while (entire pattern found or end of text)
tetththeheehthtehtheththehehtht the tetththeheehthtehtheththehehtht the tetththeheehthtehtheththehehtht the tetththeheehthtehtheththehehtht the tetththeheehthtehtheththehehtht the tetththeheehthtehtheththehehtht the
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Brute Force-Complexity • Given a pattern M characters in length, and a text N characters in length... • Worst case: compares pattern to each substring of text of length M. For example, M=5.
1) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 5 comparisons made 2) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 5 comparisons made 3) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 5 comparisons made 4) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 5 comparisons made 5) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 5 comparisons made .... N) AAAAAAAAAAAAAAAAAAAAAAAAAAAH 5 comparisons made AAAAH
• Total number of comparisons: M (N-M+1) • Worst case time complexity: Ο(MN) Strings and Pattern Matching
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Brute Force-Complexity(cont.) • Given a pattern M characters in length, and a text N characters in length... • Best case if pattern found: Finds pattern in first M positions of text. For example, M=5.
1) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAA 5 comparisons made
• Total number of comparisons: M • Best case time complexity: Ο(M)
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Brute Force-Complexity(cont.) • Given a pattern M characters in length, and a text N characters in length... • Best case if pattern not found: Always mismatch on first character. For example, M=5. 1) AAAAAAAAAAAAAAAAAAAAAAAAAAAH OOOOH 1 comparison made 2) AAAAAAAAAAAAAAAAAAAAAAAAAAAH OOOOH 1 comparison made 3) AAAAAAAAAAAAAAAAAAAAAAAAAAAH OOOOH 1 comparison made 4) AAAAAAAAAAAAAAAAAAAAAAAAAAAH OOOOH 1 comparison made 5) AAAAAAAAAAAAAAAAAAAAAAAAAAAH OOOOH 1 comparison made ... N) AAAAAAAAAAAAAAAAAAAAAAAAAAAH 1 comparison made OOOOH • Total number of comparisons: N • Best case time complexity: Ο(N) Strings and Pattern Matching
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Rabin-Karp • The Rabin-Karp string searching algorithm uses a hash function to speed up the search.
Rabin & Karp’s
Heavenly Homemade Hashish Fresh from Syria
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Rabin-Karp • The Rabin-Karp string searching algorithm calculates a hash value for the pattern, and for each M-character subsequence of text to be compared. • If the hash values are unequal, the algorithm will calculate the hash value for next M-character sequence. • If the hash values are equal, the algorithm will do a Brute Force comparison between the pattern and the M-character sequence. • In this way, there is only one comparison per text subsequence, and Brute Force is only needed when hash values match. • Perhaps a figure will clarify some things...
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Rabin-Karp Example Hash value of “AAAAA” is 37 Hash value of “AAAAH” is 100
1) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 37≠100 1 comparison made 2) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 37≠100 1 comparison made 3) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 37≠100 1 comparison made 4) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 37≠100 1 comparison made ... N) AAAAAAAAAAAAAAAAAAAAAAAAAAAH AAAAH 6 comparisons made 100=100
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Rabin-Karp Pseudo-Code pattern is M characters long hash_p=hash value of pattern hash_t=hash value of first M letters in body of text do if (hash_p == hash_t) brute force comparison of pattern and selected section of text hash_t = hash value of next section of text, one character over while (end of text or brute force comparison == true)
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Rabin-Karp • Common Rabin-Karp questions: “What is the hash function used to calculate values for character sequences?” “Isn’t it time consuming to hash every one of the M-character sequences in the text body?” “Is this going to be on the final?”
• To answer some of these questions, we’ll have to get mathematical.
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Rabin-Karp Math • Consider an M-character sequence as an M-digit number in base b, where b is the number of letters in the alphabet. The text subsequence t[i .. i+M-1] is mapped to the number x(i) = t[i]⋅bM-1 + t[i+1]⋅bM-2 +...+ t[i+M-1] • Furthermore, given x(i) we can compute x(i+1) for the next subsequence t[i+1 .. i+M] in constant time, as follows: x(i+1) = t[i+1]⋅bM-1 + t[i+2]⋅bM-2 +...+ t[i+M] x(i+1) = x(i)⋅b
Shift left one digit
- t[i]⋅b M
Subtract leftmost digit
+ t[i+M]
Add new rightmost digit
• In this way, we never explicitly compute a new value. We simply adjust the existing value as we move over one character.
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Rabin-Karp Mods • If M is large, then the resulting value (~bM) will be enormous. For this reason, we hash the value by taking it mod a prime number q. • The mod function (% in Java) is particularly useful in this case due to several of its inherent properties: - [(x mod q) + (y mod q)] mod q = (x+y) mod q - (x mod q) mod q = x mod q • For these reasons: h(i) = ((t[i]⋅ bM-1 mod q) + (t[i+1]⋅ bM-2 mod q) + ... + (t[i+M-1] mod q)) mod q
h(i+1) =( h(i)⋅ b mod q Shift left one digit -t[i]⋅ bM mod q Subtract leftmost digit +t[i+M] mod q ) Add new rightmost digit mod q
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Rabin-Karp Pseudo-Code pattern is M characters long hash_p=hash value of pattern hash_t =hash value of first M letters in body of text do if (hash_p == hash_t) brute force comparison of pattern and selected section of text hash_t = hash value of next section of text, one character over while (end of text or brute force comparison == true)
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Rabin-Karp Complexity • If a sufficiently large prime number is used for the hash function, the hashed values of two different patterns will usually be distinct. • If this is the case, searching takes O(N) time, where N is the number of characters in the larger body of text. • It is always possible to construct a scenario with a worst case complexity of O(MN). This, however, is likely to happen only if the prime number used for hashing is small.
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The Knuth-Morris-Pratt Algorithm • The Knuth-Morris-Pratt (KMP) string searching algorithm differs from the brute-force algorithm by keeping track of information gained from previous comparisons. • A failure function (f) is computed that indicates how much of the last comparison can be reused if it fais. • Specifically, f is defined to be the longest prefix of the pattern P[0,..,j] that is also a suffix of P[1,..,j] - Note: not a suffix of P[0,..,j] • Example: - value of the KMP failure function: j
0
1
2
3
4
5
P[j]
a
b
a
b
a
c
f(j)
0
0
1
2
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0
• This shows how much of the beginning of the string matches up to the portion immediately preceding a failed comparison. - if the comparison fails at (4), we know the a,b in positions 2,3 is identical to positions 0,1 Strings and Pattern Matching
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The KMP Algorithm (contd.) • Time Complexity Analysis • define k = i - j • In every iteration through the while loop, one of three things happens. - 1) if T[i] = P[j], then i increases by 1, as does j k remains the same. - 2) if T[i] != P[j] and j > 0, then i does not change and k increases by at least 1, since k changes from i - j to i - f(j-1) - 3) if T[i] != P[j] and j = 0, then i increases by 1 and k increases by 1 since j remains the same. • Thus, each time through the loop, either i or k increases by at least 1, so the greatest possible number of loops is 2n • This of course assumes that f has already been computed. • However, f is computed in much the same manner as KMPMatch so the time complexity argument is analogous. KMPFailureFunction is O(m) • Total Time Complexity: O(n + m) Strings and Pattern Matching
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The KMP Algorithm (contd.) • the KMP string matching algorithm: Pseudo-Code Algorithm KMPMatch(T,P) Input: Strings T (text) with n characters and P (pattern) with m characters. Output: Starting index of the first substring of T matching P, or an indication that P is not a substring of T. f ← KMPFailureFunction(P) {build failure function} i←0 j←0 while i < n do if P[j] = T[i] then if j = m - 1 then return i - m - 1 {a match} i←i+1 j←j+1 else if j > 0 then {no match, but we have advanced} j ← f(j-1) {j indexes just after matching prefix in P} else i←i+1 return “There is no substring of T matching P”
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The KMP Algorithm (contd.) • The KMP failure function: Pseudo-Code Algorithm KMPFailureFunction(P); Input: String P (pattern) with m characters Ouput: The faliure function f for P, which maps j to the length of the longest prefix of P that is a suffix of P[1,..,j] i←1 j←0 while i ≤ m-1 do if P[j] = T[j] then {we have matched j + 1 characters}
f(i) ← j + 1 i←i+1 j←j+1 else if j > 0 then
{j indexes just after a prefix of P that matches}
j ← f(j-1) else
{there is no match}
f(i) ← 0 i←i+1
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The KMP Algorithm (contd.) • A graphical representation of the KMP string searching algorithm a
b a c a
a b a
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a
b a c a
b
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c c a
b a
c a b a
a
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a
no comparison needed here
b a c a
b
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a
b a c a
b
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a
b a c a
b
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a
Strings and Pattern Matching
b a c a
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Regular Expressions • notation for describing a set of strings, possibly of infinite size • ε denotes the empty string • ab + c denotes the set {ab, c} • a* denotes the set {ε, a, aa, aaa, ...} • Examples - (a+b)* all the strings from the alphabet {a,b} - b*(ab*a)*b* strings with an even number of a’s - (a+b)*sun(a+b)* strings containing the pattern “sun” - (a+b)(a+b)(a+b)a 4-letter strings ending in a
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Finite State Automaton • “machine” for processing strings a 0
1
a b
a
0
b
b
1
a
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a 2
ε
a
ε ε
ε
1
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ε 4
Strings and Pattern Matching
ε b
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b
a,b
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Composition of FSA’s ε a
ε
ε
α
α
β
ε
ε
ε
β
ε
α
ε Strings and Pattern Matching
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Tries • A trie is a tree-based date structure for storing strings in order to make pattern matching faster. • Tries can be used to perform prefix queries for information retrieval. Prefix queries search for the longest prefix of a given string X that matches a prefix of some string in the trie. • A trie supports the following operations on a set S of strings: insert(X): Insert the string X into S Input: String Ouput: None remove(X): Remove string X from S Input: String Output: None prefixes(X): Return all the strings in S that have a longest prefix of X Input: String Output: Enumeration of strings
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Tries (cont.) • Let S be a set of strings from the alphabet Σ such that no string in S is a prefix to another string. A standard trie for S is an ordered tree T that: - Each edge of T is labeled with a character from Σ - The ordering of edges out of an internal node is determined by the alphabet Σ - The path from the root of T to any node represents a prefix in Σ that is equal to the concantenation of the characters encountered while traversing the path. • For example, the standard trie over the alphabet Σ = {a, b} for the set {aabab, abaab, babbb, bbaaa, bbab} a
b
a
b
b
a
a
a b
a
b 1
b a
b b
a b
2
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b a
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Tries (cont.) • An internal node can have 1 to d children when d is the size of the alphabet. Our example is essentially a binary tree. • A path from the root of T to an internal node v at depth i corresponds to an i-character prefix of a string of S. • We can implement a trie with an ordered tree by storing the character associated with an edge at the child node below it.
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Compressed Tries • A compressed trie is like a standard trie but makes sure that each trie had a degree of at least 2. Single child nodes are compressed into an single edge. • A critical node is a node v such that v is labeled with a string from S, v has at least 2 children, or v is the root. • To convert a standard trie to a compressed trie we replace an edge (v0, v1) each chain on nodes (v0, v1...vk) for k 2 such that - v0 and v1 are critical but v1 is critical for 0 1 do f1 ← Q.minKey() T1 ← Q.removeMinElement() f2 ← Q.minKey() T2 ← Q.removeMinElement() Create a new tree T with left subtree T1 and right subtree T2. Q.insertItem(f1 + f2) return tree Q.removeMinElement()
• runing time for a text of length n with k distinct characters: O(n + k log k) Strings and Pattern Matching
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Image Compression • we can use Huffman encoding also for binary files (bitmaps, executables, etc.) • common groups of bits are stored at the leaves • Example of an encoding suitable for b/w bitmaps
0 0
1 0
1
000
1
111 0
1
010
101
Strings and Pattern Matching
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011
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